This is a proposal to examine one of the mechanisms which could explain how epizootic VEE virus persists in nature between the interepizootic periods. Mares free of VEE antibodies will be infected with an epizootic VEE virus (MF-8 strain) at different times during their gestational period. Mares fit to breed that survive the infection will be left to deliver and their foals will be tested for persistence of latent infection, following a series synthetic corticosteroids. Presence of VEE virus in organs will be determined by organ explant culture technique. Surviving dams and colts will be examined for latent virus for periods up to 2 years after infection. The efficiency to infect Culex (Melanoconion) aikeniii mosquitoes by those animals which show cycles of reappearance of virus will be investigated. Insect-proof animal quarters will be available in a pasture some 100 Km west of the Pacific entrance to the Panama Canal. The area is about 900 meters above sea level.